The process for synthesizing polycarbonate by the interfacial polymerization process is described variously in the literature, for example inter alia in Schnell, “Chemistry and Physics of Polycarbonates”, Polymer Reviews, Volume 9, Interscience Publishers, New York, London, Sydney 1964, pp. 33–70.
In the interfacial polymerization process the phosgenation of a disodium salt of a bisphenol (or a mixture of various bisphenols) in aqueous-alcoholic solution (or suspension) is carried out in the presence of an inert organic solvent or solvent mixture that forms a second phase. The oligocarbonates that are formed, and that are present mainly in the organic phase, undergo condensation by means of suitable catalysts to form high molecular weight polycarbonates dissolved in the organic phase. The organic phase is finally separated.
The polycarbonate must then be isolated from the organic phase. The current methods for concentrating the polycarbonate solution and for isolating the polycarbonate are described in the patent literature and in text books and are known to the person skilled in the art. The isolation of the polycarbonate from the solution is preferably carried out by evaporating the solvent by heating or applying a vacuum. This process requires the use of a high boiling point (>100° C.) solvent, for example chlorobenzene, in order directly to obtain the melt phase after the evaporation of the solvent. In order to improve the solubility of the polymer in the solvent during the reaction, a mixture of one or more high boiling point solvents and the low boiling point compound dichloromethane is also used.
Typically the weight ratio of dichloromethane to the high boiling point solvent is about 1:1.
Another possibility is to inject a heated gas, in particular steam, in order to expel the volatile constituents. In this case the polycarbonate solution is sprayed together with the carrier gas and polycarbonate is formed as a solid, especially as a water-wet suspension. Other isolation methods include crystallization and precipitation as well as heating the residues of the solvent in the solid phase. The last method requires the use of dichloromethane as solvent, whereby residual contents of volatile constituents of about 2 ppm dichloromethane may be achieved. Residual contents of dichloromethane particularly interfere in the polycarbonate since dichloromethane with residual moisture together split off hydrogen chloride during plastics processing and may thus cause discolorations of the polycarbonate and lead to corrosion of the equipment.
In the known methods for evaporation, or also flash evaporation, polycarbonate solutions are repeatedly heated under slight excess pressure to temperatures above the boiling point and these superheated solutions are then flashed into a vessel at a lower pressure than the vapor pressure in the solution. The evaporation or flash evaporation of the solvent may be carried out using various methods, equipment and machinery, e.g. stripping extruders, thin-film evaporators or friction-compaction devices. Conventional methods for the apparatus-based evaporation of polycarbonate solutions are known to the person skilled in the art. For example the superheated solution may be flashed into a heated coil evaporator that terminates in a separator. In this connection it may be advantageous to carry out the process in several stages. In addition processes are known in which in some cases a multistage concentration of the solution is carried out in vertical shell-and-tube heat exchangers.
In order to achieve particularly low contents of residuals falling strand evaporators may be used for the last degassing stage. In this case the polymer melt is formed into thin strands in a separator under a vacuum and elevated temperature and thereby freed from the solvent. The disadvantage of the falling strand technique is that an effective degassing is ensured only by stable strands, i.e. strands that do not break off in the equipment. The stability of the strands is influenced by the viscosity of the polymer solution. Too low a viscosity may lead to strand breakages, which in turn means a restriction of the operating parameters in terms of temperature and entry content of residual volatile constituents. Apart from the negative influence on the viscosity, too high an entry concentration of volatile constituents has direct deleterious effects on the degree of degassing that may be achieved, since the mass transfer is determined purely by diffusion. The surface available for the mass transfer is however fixed by the strand geometry. The necessary large area of the melt distributor that is needed to produce the strands requires in turn large expensive equipment.
It is also known from the prior art to carry out the last degassing step in the working-up of polymers under foaming (expansion) with or without entrainment agents. Methods for the apparatus-based degassing of thermoplastic polymers that are based on the principle of foam degassing exist in the prior art. However, these methods often have the disadvantage that either very large amounts of gas have to be removed, or that sufficiently low content of residuals cannot be achieved. Furthermore, the methods described in the literature are often inflexible as regards throughput and polymer type and viscosity, and as regards the entry concentration of volatile components at the inlet to the respective degassing stage.
EP-A 200 368 describes the foam degassing of styrene polymers in a two-stage process. The method is based on the fact that the amount of solvent in the polymer solution is adjusted to be sufficiently large so as to initiate a foaming.
EP-A 914 355 describes mixing a sparingly soluble separating agent in a polymer solution followed by flashing, with foaming, into a separator under low pressure. The process described there is used to separate readily volatile solvents with the aid of foaming agents. The readily volatile separating agent is not completely dissolved, but is dispersed.
DE-A 100 15 862 describes a multistage process for removing volatile constituents from polyamides. Before the last degassing stage foaming agents for this, e.g. nitrogen, carbon dioxide or water, are dispersed using a static mixer and thereby partially dissolved. The polyamide melt is then flashed in a vertical shell-and-tube heat exchanger or in a loop evaporator or in a combination thereof into a degassing vessel under low pressure. Wire loops onto which the melt flows from above ensure an extended residence time and thus a better degassing result.